This module lets you group domains together. It has no other functionality, but can be used to for example show a list of domains in a country region or any other type of grouping you might think of. The module uses the Export UI from CTools, so groups should be exportable. Not tested though.
A submodule, Domain Groups Mappings, is included that lets you map Domain Groups to one or more domains. You could use this to create filters that varies depending on what domain you're visiting, for example.